High Power Crystal SetAugust
1960 Popular Electronics

Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.

Most regular RF Cafe visitors will probably not be too interested
in this article, but there are a lot of people who build and/or
repair vintage radio gear and search the Internet for helpful information.
Having built a couple crystal radio sets as a kid, I've always been
amazed at how a few picowatts of RF energy can be received, processed,
and heard through an ear plug without the need for external power
from a battery.

Speaking of crystal radios, I remember one time while working
as an electrician in Annapolis, Maryland, (prior to entering electronics)
I had a telephone handset for use in communicating with other electricians
in a building I was wiring, and it picked up the local AM radio
station. A pair of the old style handsets with carbon microphones
would, with the help of a single 'D' cell in series, function as
a very acceptable intercom system using two standard electrical
wires between them. I could go on the building roof to work on a
compressor unit and communicate with a guy in the panel room simply
by having us connect to the same two wires (usually 14 or 12 gauge).
Anyway, the microphone evidently acted as a rectifier, possibly
due to dirty spring contacts against the element, and processed
the AM radio signal. It was clear as a bell. After hearing the broadcast,
I looked around (remember I was on the roof) and saw that I was
about a block away from an AM antenna tower. Mystery solved. I never
have had a dental filling receive a radio broadcast.

Here's a pint-sized crystal radio with enough oomph to drive
a 2 1/2" speaker. This little unit's selectivity is far better than
you'd expect to find in a crystal receiver and volume is equal to
that obtained with sets using a transistor. No external power source
is required.

The unusual selectivity of this radio is due to its special double-tuned
circuit. A pair of diodes connected as a voltage-doubler provides
the extra kick to operate the small speaker. An output jack is provided
for headphone listening and for connecting the set to an amplifier.

Construction.

The model was built on a 2 1/2" x 4 1/2" wooden chassis with
a 3 1/2" x 4 1/2" metal front panel. However, size is not critical,
and other materials can be substituted if desired.

Two standard ferrite loopsticks, L2, and L3, are used. Both must
be modified by the addition of a second winding, L1 and L4, respectively.
Each of the added windings consists of 22 turns of No. 24 cotton-covered
wire wound on a small cardboard tube as shown on the pictorial.
(Actually, any wire size from No. 22 to No. 28 with cotton or enamel
insulation will do the job.) The diameter of the cardboard tube
should be slightly larger than L2 and L3 so that L1 and L4 will
slip over L2 and L3 easily.

Layout is not critical but L2 and
L3 should be mounted at right angles to each other.

The crystal set shown was built on
a wooden chassis. If a metal chassis is used,be sure to insulate
the Fahnestock clips (antenna and ground) from the chassis.

For phone operation only, the speaker, transformer, and resistor
R1 can be omitted.In this case, connect high-impedance phones
in place of R1.

Resistor R1 is used only for feeding the set into an amplifier;
it should be omitted for both earphone and loudspeaker operation.
Trimmer capacitor C2 should be soldered across the stator terminals
of two-gang variable capacitor C1a/C1b, as shown. The speaker and
output transformer can be mounted wherever convenient.

After all of the parts have been mounted on the chassis, wire them
together following the schematic and pictorial diagrams. Be sure
that diodes D1 and D2 and capacitors C3 and C4 are correctly polarized.

To align the receiver, first connect it to an antenna and ground.
(The optimum length of the antenna varies with location, but 50
feet will usually be suitable in areas serviced by several broadcast
stations.) Next, plug in a high-impedance earphone at jack J1. Tune
in a station near the high-frequency end of the broadcast band -
say 1500 kc. ­ and adjust the trimmer capacitors on variable capacitor
C1a/C1b for the loudest signal.

Trimmer capacitor C2 should then be adjusted for the best selectivity
and volume over the entire broadcast band. Finally, coils L1 and
L4 can be optimally positioned by sliding them back and forth over
coils L2 and L3. If a nearby station interferes with reception of
a weaker one, tune the slug on L2 for minimum interference.

For loudspeaker operation, simply unplug the earphone from J1
- strong local stations should come in with fair volume. To operate
the set as an AM tuner, wire R1 in place and connect J1 to the crystal-phono
input of a preamplifier or integrated amplifier. The set should
give excellent results with a quality hi-fi system.

How It Works.

The receiver employs a double-tuned circuit feeding a crystal-diode
voltage-doubler/detector which drives a small speaker. In operation,
r.f. signals picked up by the antenna system are induced into coil
L2 from coil L1. The desired signal is selected by tuned circuit
C1a-L2 and coupled through capacitor C2 to a second tuned circuit,
C1b-L3, which improves the selectivity by narrowing the r.f. bandpass.
The twice-tuned r.f. signal is then induced into coil L4 from coil
L3.

The positive half of the r.f. signal appearing across L4 passes
through diode D2 to charge capacitor C4; the negative half of the
signal passes through diode D1 to charge capacitor C3. Polarities
of the charges on C3 and C4 are such that the effective voltage
is doubled. This voltage appears across the primary of output transformer
T1, which changes the high impedance at the output of diodes D1
and D2 to the low impedance required by the speaker.

When high-impedance earphones are plugged into closed-circuit
jack J1, the speaker is disconnected and the output from the diodes
feeds directly into the earphones. Optional load resistor R1 is
placed across the output of the diodes when the receiver is used
with an amplifier.

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